Water treatment process, and membrane separation process and water treatment plant suitable therefor

ABSTRACT

A description is given of a multistage process for treating water, wherein a water stream is purified in a membrane separation stage and a downstream deionization unit having at least one concentrate chamber and at least one diluate chamber, wherein the water stream is separated in the membrane separation device into a concentrate stream and at least two permeate streams of different electrical conductivity, and wherein at least the permeate stream having the highest electrical conductivity is fed at least partially into the at least one concentrate chamber and at least the permeate stream having the lowest electrical conductivity is fed at least partially into the at least one diluate chamber of the downstream deionization unit. In addition, a membrane separation device is described which is constructed in such a manner that at least two permeate streams having different electrical conductivity can be generated therein, and also a water treatment plant, comprising at least one such membrane separation device and at least one deionization unit having at least one concentrate chamber and at least one diluate chamber.

RELATED APPLICATIONS

This is a §371 of International Application No. PCT/EP2009/007981, withan international filing date of Nov. 9, 2009, which is based on GermanPatent Application No. 10 2008 057 669.7, filed Nov. 11, 2008, thesubject matter of which is incorporated by reference.

TECHNICAL FIELD

This disclosure relates to a multistage process for treating water,wherein a water stream is purified in a membrane separation stage and adownstream deionization unit having at least one concentrate chamber andat least one diluate chamber. This disclosure also relates to a membraneseparation device which can be used in such a process, and a plant fortreating water which is suitable for carrying out the process.

BACKGROUND

The treatment of water is currently of constantly increasing importance.In addition to drinking water, especially in the chemical andpharmaceutical industries, high-purity process waters are required whichmust be prepared in a large quantity as inexpensively as possible.High-purity water, in addition, is especially also required in thesemiconductor industry, for example for rinsing silicon wafers, inparticular after etching processes. The purity requirements of the waterare known to be particularly high in this sector.

It is known that the provision of ultrapure water can be achieved with amultistage process comprising a first stage in which the raw water issoftened and/or already partially desalinated, a second stage in whichthe water from the first stage is further purified in a pressure-drivenmembrane separation process, and a third stage in which the water isfinally substantially completely deionized, for example byelectrodeionization. In addition, further process steps, in particularfor eliminating organic impurities, can be further provided.

Water softening and/or desalination in the first stage generally proceedby use of one or more ion exchangers. For the softening, cationexchangers in the sodium form are preferably used, whereas for thedesalination, combinations of cation and anion exchangers are customary.The total ionic load of the water to be treated can be markedly reducedalready by such methods.

Membrane separation processes which come into consideration are, inparticular, reverse osmosis and nanofiltration, optionally also incombination. If relatively large amounts of dissolved carbon dioxide arepresent in the raw water, this process sequence can be furthersupplemented by a degassing step before or after the membrane separationprocess.

If a high water yield is of importance, the concentrate from themembrane separation stage can be treated in a further additionalmembrane separation stage, wherein the resultant permeate generally,owing to its high electrical conductivity, cannot be directlytransferred to a deionization step. Instead, it is customarilyrecirculated and added upstream of the membrane separation stage to thewater to be treated.

Electrodeionization devices, in customary designs, always require asolution which takes up the ions that are separated off from the waterto be treated and discharges them (concentrate) from the device. Thissolution flows through at least one concentrate chamber, and the waterto be treated through at least one diluate chamber. A high ionicconductivity in the concentrate chambers in this case is known to beachieved, in particular, by the following:

(a) an addition, e.g. of neutral salts being formed,

(b) the concentrate being recirculated through the concentrate chambers,in such a manner that the ions that are separated off accumulate thereor

(c) the concentrate chambers (as also the diluate chambers) are packedwith ion-exchange resins

Alternatively, concentrate from an upstream membrane separation stagecan also be fed into the concentrate chambers of an electrodeionizationdevice. However, this must generally be worked up in an intermediatestep, as disclosed by WO 2005/113120.

It could therefore be helpful to provide a technical solution for themultistage treatment of water, which, compared with known solutions,shall have a structure kept as simple as possible, in particular asconcerns the deionization stage.

SUMMARY

We provide a multistage process for treating water including purifying awater stream in a membrane separation stage and a downstreamdeionization unit having at least one concentrate chamber and at leastone diluate chamber, separating the water stream in the membraneseparation stage into a concentrate stream and at least two permeatestreams of different electrical conductivity, and feeding a) at leastthe permeate stream having the highest electrical conductivity at leastpartially into the at least one concentrate chamber and b) at least thepermeate stream having the lowest electrical conductivity at leastpartially into the at least one diluate chamber of the downstreamdeionization unit.

We also provide a membrane separation for treating water constructedsuch that at least two permeate streams of different electricalconductivity are generated therein.

We further provide a plant for treating water including at least onemembrane separation device, at least one deionization unit having atleast one concentrate chamber and at least one diluate chamber.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows an example of our membrane separation device.

DETAILED DESCRIPTION

The process is a multistage process for treating water, in particularfor producing ultrapure water which is suitable for the applicationsmentioned at the outset. In the process, a water stream is purified in amembrane separation stage and a downstream deionization unit having atleast one concentrate chamber and at least one diluate chamber, inparticular in a corresponding electrodeionization unit.

Permeate from a membrane separation stage, depending on the feedconcentration, has a varying composition. At the inlet, the salt contentof the water to be treated is relatively low, for which reason thepermeate also has a relatively low salt content and a relatively lowelectrical conductivity. However, the further from the feed within amembrane separation stage, the stronger are the salts enriched in theconcentrate. As a consequence, the permeate which is produced by themembrane at this point of the membrane separation stage generally alsohas a higher salt content and also a higher conductivity.

We utilize this effect by feeding in low-salt permeate from one membraneseparation stage on the diluate side of the membrane to a downstreamdeionization unit, while high-salt permeate of the concentrate side isfed to the deionization unit.

The process is correspondingly distinguished in that the water stream tobe treated is separated in the membrane separation stage into aconcentrate stream and two or more permeate streams of differentelectrical conductivity. At least the permeate stream having the highestelectrical conductivity is partially or completely fed into the at leastone concentrate chamber of the downstream deionization unit. At leastthe permeate stream having the lowest electrical conductivity ispartially or completely fed into the at least one diluate chamber.

The permeate stream can be separated, for example, by contacting thewater stream in the membrane separation stage with at least two membranemodules, particularly preferably with exactly two membrane modules,arranged in such a manner that they are successively subjected toincoming flow of the water to be treated. Preferably, the membranemodules each have a separate permeate discharge.

The at least two membrane modules are preferably reverse osmosis modulesand/or nanofiltration modules. Suitable modules are described furtherhereinafter.

If, e.g., two reverse osmosis modules of the same type are arranged insuch a manner that they are successively subjected to incoming flow,then the membrane module subjected to incoming flow first generallyproduces the permeate stream having the lowest electrical conductivity.The membrane module subjected to incoming flow last produces in thiscase the permeate stream having the highest electrical conductivity. Thesame applies in principle also to nanofiltration modules of the sametype.

However, alternatively, it is also conceivable, for example, in themembrane separation stage to arrange, e.g., a nanofiltration module anda reverse osmosis module in such a manner that first the nanofiltrationmodule and then the reverse osmosis module is subjected to incomingflow. Generally, the nanofiltration module, even if it is subjected toincoming flow first, always produces the permeate stream having thehigher electrical conductivity, since it customarily does not pose agreat obstacle, at least to the monovalent ions most frequentlyoccurring, in contrast to reverse osmosis.

Correspondingly, in a combination of a nanofiltration module and areverse osmosis module, the membrane modules can be arranged inprinciple in any desired sequence, in such a manner that, e.g., areverse osmosis module is subjected to incoming flow first, and then ananofiltration module is subjected to incoming flow.

Concentrate exiting from the deionization unit, preferably is at leastpartially recirculated and added back to the water stream upstream, inparticular upstream of the membrane separation stage. The water yield ofthe overall system can thereby be increased.

Before the water stream is fed into the membrane separation stage, it ispreferably partially desalinated and/or softened by at least one ionexchanger. Suitable ion exchangers are known to those skilled in theart.

Particularly preferably, the permeate stream to be fed into the at leastone concentrate chamber of the deionization unit has an electricalconductivity in the range between 5 μS/cm and 500 μS/cm. Within thisrange, values between 10 μS/cm and 300 μS/cm, in particularapproximately 100 μS/cm, are further preferred. To achieve thisconductivity range, the permeate stream can optionally be concentratedor diluted (e.g. by increasing the fraction of more highly or lessconcentrated permeate).

The process offers in particular the following advantages:

-   -   Sufficient conductivity in the concentrate chambers of the        deionization unit can be ensured even without addition of        neutral salts or circulation of the concentrate. The expenditure        on apparatus for these measures is dispensed with.    -   Permeate of low and high conductivity can be generated in the        same pressure housing, and the construction of the membrane        separation device to be used can therefore be kept very simple.

The latter aspect in particular will be considered further hereinafter.

A membrane separation device is suitable in particular for treatingwater in the above-described process. It is constructed in such a mannerthat at least two permeate streams having different electricalconductivity can be generated therein.

Particularly preferably, the membrane separation device has at least twomembrane modules arranged in such a manner that they can be successivelysubjected to incoming flow of the water to be treated as alreadymentioned above.

Likewise, it has already been mentioned that the membrane modules can benot only nanofiltration modules but also reverse osmosis modules,optionally also combinations of both in any desired sequence.

The at least two membrane modules can be structurally separated from oneanother and each accommodated in separate containers, optionally, also apump can be arranged between the membrane modules.

Particularly preferably, a membrane separation device comprises,however, two or more membrane modules arranged within a shared pressurevessel. This is preferred, in particular, when the membrane modules arereverse osmosis modules of the same type. Suitable pressure vessels areknown and they are generally customary vessels for receiving membranemodules for reverse osmosis.

Particularly preferably, the pressure vessel is a simple tube in whichthe at least two membrane modules are arranged one behind the other inan axial direction.

Suitable membrane modules are, for example, spirally wound modules.These can have one or more membrane pockets which, together with anet-like spacer, are spirally wound around a perforated permeatecollecting tube. The membrane pockets in this case preferably comprisetwo membranes between which the spacer mentioned is arranged. Thepockets are closed on three sides and connected to the permeatecollecting tube at the fourth open side. Water to be treated flowsthrough such a module in an axial direction, while the permeate flows tothe collecting tube in a spiral manner.

Particularly preferably, the at least two membrane modules of a membraneseparation device each have a separate permeate outflow. Via these, thepermeate streams of different electrical conductivity can then be fed totheir respective destination.

A plant for treating water comprises at least one membrane separationdevice as has already been described above, and also at least onedeionization unit, in particular at least one electro-deionization unit,having at least one concentrate chamber and at least one diluatechamber. In particular, the plant is suitable for carrying out a watertreatment process.

Similarly thereto, in the at least one membrane separation device of theplant, preferably at least two membrane modules are arranged in such amanner that they can be successively subjected to incoming flow of thewater to be treated. The at least two membrane modules each have aseparate permeate outflow. At the same time, at least the permeateoutflow of the membrane module subjected to incoming flow first iscoupled to the at least one diluate chamber, and at least the permeateoutflow of the membrane module subjected to incoming flow last iscoupled to the at least one concentrate chamber. This can be the case,in particular, when the at least two membrane modules are either solelynanofiltration modules of preferably the same type, or solely reverseosmosis modules of preferably the same type.

Alternatively, in the at least one membrane separation device of theplant, preferably at least two membrane modules are arranged in such amanner that they can be successively subjected to incoming flow of thewater to be treated, wherein the at least two membrane modules each havea separate permeate outflow.

Preferably, in particular when the at least two membrane modules areeither reverse osmosis modules or nanofiltration modules in each case ofthe same type, the permeate outflow of the membrane module subjected toincoming flow first is coupled to the at least one diluate chamber, andthe permeate outflow of the membrane module subjected to incoming flowlast is coupled to the at least one concentrate chamber. Furtherpreferably, the membrane module subjected to incoming flow first is areverse osmosis module and the membrane module subjected to incomingflow last is a nanofiltration module.

Further preferably, the permeate outflow of the membrane modulesubjected to incoming flow first is coupled to the at least oneconcentrate chamber, and the permeate outflow of the membrane modulesubjected to incoming flow last is coupled to the at least one diluatechamber. This is the case, in particular, when the membrane modulesubjected to incoming flow first is a nanofiltration module and themembrane module subjected to incoming flow last is a reverse osmosismodule.

Further features result from the description of the drawing hereinafter.The individual features can each be implemented alone, or a plurality incombination with one another can be implemented. The drawing servesmerely for illustration and for better understanding, and is in no wayto be taken to be restricting.

FIG. 1 shows an example of our membrane separation device. What is shownis the pressure tube 100 in which the membrane modules 101 a and 101 bare arranged one behind the other. Both modules are spirally woundmodules. Water to be treated enters into the pressure tube 100 via theintake 102 and then the two membrane modules 101 a and 101 b aresubjected successively to incoming flow. In the membrane module 101 a,correspondingly permeate having a low salt content and a low electricalconductivity is generated, in the membrane module 101 b, permeate havinga higher salt content and also a higher electrical conductivity isgenerated. The concentrate is finally removed via the outlet 103. Eachof the membrane modules 101 a and 101 b has a separate outflow for thepermeate generated, namely outflows 104 a and 104 b. The component 105in classical membrane separation devices is a connection between thepermeate collecting tubes of the membrane modules. In the present case,however, this connection is blocked, ultimately mixing of the permeateobtained in the modules 101 a and 101 b is not wanted. This component105 therefore functions merely as a spacer.

The invention claimed is:
 1. A multistage process for treating watercomprising: purifying a water stream in a membrane separation stage anda downstream deionization unit having at least one concentrate chamberand at least one diluate chamber; separating the water stream in themembrane separation stage with at least two spirally wound membranemodules arranged in a tubular pressure vessel, one behind the other, inan axial direction and successively subjected to incoming flow of thewater to be treated into a concentrate stream and at least two permeatestreams of different electrical conductivity; and feeding a) at leastthe permeate stream having the highest electrical conductivity at leastpartially into the at least one concentrate chamber and b) at least thepermeate stream having the lowest electrical conductivity at leastpartially into the at least one diluate chamber of the downstreamdeionization unit.
 2. The process as claimed in claim 1, wherein the atleast two membrane modules are reverse osmosis modules and/ornanofiltration modules.
 3. The process as claimed in claim 1, whereinconcentrate exiting from the deionization unit is at least partiallyrecirculated and added back to the water stream upstream of the membraneseparation stage.
 4. The process as claimed in claim 1, wherein thewater stream, before it is fed into the membrane separation stage, ispartially desalinated and/or softened by at least one ion exchanger. 5.The process as claimed in claim 1, wherein the permeate stream fed intothe at least one concentrate chamber of the deionization unit isadjusted to an electrical conductivity of 5 μS/cm to 500 μS/cm.
 6. Theprocess as claimed in claim 1, wherein the permeate stream fed into theat least one concentrate chamber of the deionization unit is adjusted toan electrical conductivity of 10 μS/cm to 300 μS/cm.